Facebook is facing intense political fallout and thorny legal questions a day after confirming that Russian funds paid for advertising on the social media platform aimed at influencing voters during last year’s presidential election.Mark Warner, the vice chairman of the Senate Intelligence Committee, said Thursday he hopes to call executives from Facebook, Twitter and other social media companies to testify publicly about what role their companies may have played, however unwittingly, in the wider Kremlin effort to manipulate the 2016 White House race.